HOW PLANTS GROW. 601 



makes sulphate of ammonia, which does not fly off so fast, and 

 the valuable ammonia is kept in our manure pile. 



What is Manure? In our common language, manure is 

 made to mean the droppings of our domestic animals, which all 

 careful farmers save and apply to the land for the growth of 

 crops. 



Let us inquire why these animal droppings are of so much 

 value to our crops. When we burn a plant, we have left in 

 the ashes the plant food which came from the soil. The same 

 thing happens to a less extent when the crops are eaten by ani- 

 mals. The animals use up, or burn, that is, oxidize, the parts 

 which the plant got from the air. In burning these things in a 

 fire, we make heat, and in burning them by the aid of the oxy- 

 gen taken into the lungs of animals, we also make heat, the 

 animal heat which is necessary to animal life. The droppings 

 of animals, then, contain the ash elements of the plant, with a 

 lot of woody fibre, and other indigestible things, with a quantity 

 of nitrogen in the shape of ammonia. We see, then, that the 

 droppings of our domestic animals contain all" the plant food 

 which plants got from the soil, and also a large amount of car- 

 bonaceous matter, which originally came from the air, but which 

 is always useful in the soil in aiding in the decomposition of 

 other matters, and the holding of substances which plants get 

 from the soil. We see, then, that the droppings of domestic 

 animals make the cheapest form of plant food we can supply to 

 the soil, and we should not allow any of these to be wasted. 



We find however, that, although all the elements of plant 

 food are found in animal droppings, our animals use a large part 

 of these things for other purposes. They use phosphoric acid 

 in the shape of phosphate of lime, to make their bones, and 

 they use nitrogen in making flesh. The milk of cows carries 

 off other elements of plant food. Animals thus, in consuming 

 crops, do not return to the soil all that came from it, even if all 

 their droppings are saved and returned to the land. So that 

 animals, fed entirely on the crops that grow on a farm, will 

 gradually reduce the plant food in the soil, though not so rap- 

 idly as when the crops are sold off the farm. We then see why 

 it is necessary for farmers, on most of our soils, to seek this 

 plant food elsewhere, because of the impossibility of returning, 



